Abstract: The structures of ammonium 3,5-di­nitro­benzoate, NH4+·C7H3N2O6-, (I), ammonium 4-nitro­benzoate dihydrate, NH4+·C7H4NO4-·2H2O, (II), and ammonium 2,4-di­chloro­benzoate hemihydrate, NH4+·C7H3Cl2O2-·0.5H2O, (III), have been determined and their hydrogen-bonded structures are described. All three salts form hydrogen-bonded polymeric structures, viz. three-dimensional in (I) and two-dimensional in (II) and (III). With (I), a primary cation-anion cyclic association is formed [graph set R43(10)] through N-HO hydrogen bonds, involving a carboxyl­ate group with both O atoms contributing to the hydrogen bonds (denoted O,O'-carboxyl­ate) on one side and a carboxyl­ate group with one O atom involved in two hydrogen bonds (denoted O-carboxyl­ate) on the other. Structure extension involves N-HO hydrogen bonds to both carboxyl­ate and nitro O-atom acceptors. With structure (II), the primary inter-species inter­actions and structure extension into layers lying parallel to (001) are through conjoined cyclic hydrogen-bonding motifs, viz.R43(10) (one cation, an O,O'-carboxyl­ate group and two water mol­ecules) and centrosymmetric R42(8) (two cations and two water mol­ecules). The structure of (III) also has conjoined R43(10) and centrosymmetric R42(8) motifs in the layered structure but these differ in that the first motif involves one cation, an O,O'-carboxyl­ate group, an O-carboxyl­ate group and one water mol­ecule, and the second motif involves two cations and two O-carboxyl­ate groups. The layers lie parallel to (100). The structures of salt hydrates (II) and (III), displaying two-dimensional layered arrays through conjoined hydrogen-bonded nets, provide further illustration of a previously indicated trend among ammonium salts of carb­oxy­lic acids, but the anhydrous three-dimensional structure of (I) is inconsistent with that trend.

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